The present invention relates to automatic scanning equipment and techniques, and is more particularly directed to a scanner that employs machine vision to check a sheet metal part or the like for accuracy of punched or drilled holes or other structure. The invention is more specifically concerned with a scanner device and technique in which a metal sheet can be scanned and then compared to an existing drawing or blue-print, which can be in CAD or in digitized form, for ascertaining hole locations and tolerances. The invention is also concerned with scanning devices in which the part can be scanned to produce a blue-print or drawing directly, or on which a blue-print can be scanned using machine vision.
In many manufacturing processes it is necessary to assure the accuracy of dimensions of parts and subassemblies prior to further assembly, and this is particularly true for workpieces or parts that are formed of sheet metal, where holes and cutouts must be in precise locations and must be specific sizes and shapes, within very narrow tolerances.
An automated scanning device is used for this purpose, in which the part to be checked is illuminated and a video image is processed to obtain the required measurement data. In many cases, a laser beam or plural laser beams are swept across the part, and the device picks up either the light reflected from the part or the light that passes through it. In either case, the laser has to be oriented at an angle to the part for capturing portions that are away from the central axis of the scanning device. This produces distortion in images of apertures or of edges, especially if the part has a finite thickness. Other scanning devices employ a video camera that captures an image of the part, or in some cases is moved in an X,Y raster pattern. For these purposes, the part is laid upon a light table, and the camera or other imager is positioned above the light table. In these systems, the camera and the movable carriage are exposed to dust and other atmospheric contaminates that one would expect to find associated with any sheet metal manufacturing process.